Ethalfluralin is the commercial name of a herbicide, which is widely used in agriculture and whose chemical name is 4-trifluoromethyl-2,6-dinitro-N-ethyl-N-methallylaniline or (IUPAC) N-ethyl- alpha-alpha-alpha-trifluoro-N-(methylallyl)-2,6-dinitro-p-toluidine.
Ethalfluralin belongs to a wide class of dinitro anilines having herbicidal activity and in such a class is characterized by the presence of an unsaturated methallyl or 2-methyl-2-propenyl group bound to the amino group.
It is known that crude dinitroanilines contain small amounts of nitrosamines, typically from a few ppm to a few hundred ppm. It is assumed that the origin of nitrosamines in dinitroanilines derives from the final stage of the conventional process for preparing dinitroanilines, which consists of reacting the dinitro-chloro-benzene derivative with the desired secondary amine. The dinitro-chloro-benzene derivative may contain small amounts of nitrosating agents, such as nitrogen oxides, which are present as the result of the nitration reaction. The nitrosating agents may react with the amine to form nitrosamines, whose presence in the dinitroanilines is considered undesirable due to the discovery of their carcinogenic properties in animals.
Following the development of highly sensitive methods of analysis, it is possible to ascertain the presence of nitrosamines in dinitroanilines down to a level of 0.02 ppm. The maximum level desired in a commercial product is presently defined at 0.5 ppm.
In order to reduce the amount of nitrosamines, several purification processes have been proposed. Particularly, U.S. Pat. No. 4,226,789 describes a process of treating dinitroanilines with HCl in an aqueous solution, having a concentration of at least 20%, or with gaseous HCl under low pressure. With reference specifically to ethalfluralin, U.S. Pat. No. 4,226,789 teaches the need for using gaseous hydrochloric acid under pressure, since the use of an aqueous solution of hydrochloric acid causes addition reactions to occur across the methallyl double bond resulting in the formation of the chloro derivative and reduction of the titer of ethalfluralin and the overall process yield. Furthermore, the compound produced by the addition of hydrochloric acid to ethalfluralin is stable, and its conversion to ethalfluralin by means of a treatment with sodium hydroxide is not possible.
The treatment with gaseous hydrochloric acid may not be conveniently carried out as an industrial process, since it involves substantial difficulties relating to the availability on the market and preparation of the gaseous hydrochloric acid as well as a substantial increase in the plant investment costs.
U.S. Pat. No. 4,874,895 describes a process for purifying trifluralin (4-trifluoromethyl-2,6-dinitro-N,N-di-n-propylaniline) by using aqueous hydrobromic acid in the presence of additives consisting of sulfamic acid and bisulfite. The disclosed process is advantageous since it is adapted not only for the removal of the nitrosamines, but also to avoid the growth of nitrosamines in the purified trifluralin. Moreover, the process allows the almost quantitative recovery of the treatment solution and its recycling for several purification cycles.
As compared with the several trifluralin purification processes described in literature wherein the processes are generally applicable to the whole class of the dinitroanilines, the process disclosed in U.S. Pat. No. 4,874,895 is strictly limited to trifluralin. Furthermore, in view of the teaching provided by U.S. Pat. No. 4,226,789, the treatment with aqueous halogenhydric acids was not considered as industrially applicable for the purification of unsaturated dinitroanilines.